In the typical ring oscillator configuration, three inverters operating in a current starved mode are connected in a ring. The gate capacitances of the inverters are sequentially charged and discharged. Decreasing the peak available charging current increases the time to charge and discharge the gate capacitance and thus decreases the frequency of the generated periodic output waveform. Current starving the inverters in the ring slows down the output waveform edges of the inverters and keeps the transistors forming the inverters operating in a linear analog region. A linear analog oscillation mode exists if the ring oscillator operates in the linear region all of the time, if the electric length around the ring equals zero degrees, and if the gain around the ring is greater than zero dB. When the linear analog oscillation mode exists, the output periodic waveform of the ring oscillator exhibits a second higher oscillation frequency or non-harmonically related spurs or jitter. This phenomenon has been termed "moding" in the industry. Further, the two modes of oscillation at two frequencies may exhibit an injection locking condition that results in unpredictable behavior in the output waveform frequency.
Because ring oscillators are used as the voltage controlled oscillator in phase locked loop circuits commonly used in many industrial, automotive, and telecommunications applications, unpredictable behaviors such as moding and injection locking are highly undesirable.